异常中的字符串损坏？

Question

我抛出一些解析异常。但一个例外需要破碎的字符串..？

//Parse exception 
class ParseException : public exception { 
public: 
    //Constructor 
    //Arguments: 
    // Str: Message 
    // Pos: Position 
    ParseException(string Str, int Pos) { 
     msg = Str; 
     pos = Pos; 
    } 

    //Get what happened(From exception) 
    //Returns: 
    // Message with position 
    virtual const char* what() const throw() { 
     string str = msg; 
     str += " at " + to_string(pos); 
     return str.c_str(); 
    } 
private: 
    string msg; //Message 
    int pos; //Position 
}; 

这是异常类。我扔这个例外是这样的：

throw ParseException("Mismatched bracket", P.Pos); 

，抛出此异常并进入：

try { 
    Parse(p); 
} 
catch (ParseException e) { // <<< Here 
    cerr << "ParseException: " << e.what() << endl; 
} 

而我得到的是：

ParseException: ▌▌▌▌▌▌▌▌▌▌▌▌▌▌▌▌▌▌▌▌▌▌▌▌▌▌▌▌▌▌▌▌▌▌▌▌ 

有我的代码有问题？或Visual Studio（或编译器）的问题？

Answer 1

As noted in the comments，您不能返回本地string的c_str而不会创建未定义的行为。在异常本身上存储what值的string缓存可能是有意义的; the char* returned by what needs to live as long as the exception object，所以缓存异常是合理的。

class ParseException : public exception { 
public: 
    //Constructor 
    //Arguments: 
    // Str: Message 
    // Pos: Position 
    ParseException(string Str, int Pos) : msg(std::move(Str)), pos(Pos) {} 

    //Get what happened(From exception) 
    //Returns: 
    // Message with position 
    virtual const char* what() const throw() { 
     // Lazily populate what so it's not populated until called 
     if (_what.empty()) { 
      _what = msg + " at " + to_string(pos); 
     } 
     return _what.c_str(); 
    } 
private: 
    string msg; //Message 
    int pos; //Position 
    string _what; 
}; 

另外，也计算在构造函数中的值，以便保持what兼容nothrow（匹配的C++标准库例外）。